In certain fields, particularly but not exclusively in railway signaling and automatic train control systems, there exists the essential requirement that electrical and electronic equipment be fail-safe which somtimes is alternatively referred to as "vital" operation. That is to say, in so far as is possible, it is mandatory that any conceivable failure or combination of related failures of any elements or components in the equipment or apparatus shall not give rise to a dangerous or unsafe condition. Purely by way of example, it is well known to arrange track circuit equipment for controlling railway signals in such a way that any failure, such as a short-circuit or open-circuit at any point in the arrangement, will cause the signals automatically to go to red stop or exhibit a "danger" signal. It is also well known to arrange components to be fail-safe per se; for example, it is common practice to provide that mating relay contacts respectively be constructed of carbon and silver in order to obviate contact welding.
As automatic train control equipment has developed, it has now become apparent that there is a necessity in certain circumstances for ensuring that a transformer circuit always presents a certain minimum load, and moreover that such a load must be fail-safe. It is quite obvious that merely connecting a resistor across all or part of a winding of the transformer is insufficient to ensure fail-safe or vital requirements since a lead of the resistor can become detached or the resistor could become open-circuited. Accordingly, it was necessary to devise a high integrity transformer load arrangement in order to satisfy the above necessity. The load arrangement in its various possible forms has wide applicability, as will subsequently be detailed, and may be employed in circumstances unconnected with railway systems and operation.